The present invention relates, in general, to optical crystals and more particularly to a method and apparatus for protecting polished surfaces of such crystals.
It is known that polished optical surfaces on hygroscopic crystals such as are used in electro-optic light modulators or other laser light applications, for example, can deteriorate very quickly when exposed to a humid atmosphere and, in the past, protection of such surfaces has been very difficult. Typically, a polished crystal has been protected by submerging it in an index-matching oil contained in a sealed liquid cell so that the oil prevents ambient humid air from reaching the crystal surface. Such a liquid cell has windows that allow light to reach the crystal, and the external surfaces of such windows typically are provided with anti-reflection coatings. Because electric fields are usually also applied to such crystals by way of electrodes on the crystals, such prior liquid-filled cells must be constructed to provide for such electrodes and for their corresponding electrical leads which must be brought into and out of the cells through the respective cell walls. Leak-proofing such a cell is difficult.
In addition, because light passing to and from the submerged optical crystal passes through the liquid cell, optical absorption in the liquid path produces the well-known "thermal blooming effect." As a result, maintenance of the optical beam profile at high light intensity levels is always a problem and can limit the use of such liquid cells to relatively low power levels. In spite of the foregoing problems, almost all commercial electro-optic modulators and Q-switches for lasers use one form or another of liquid cells of the same basic design.